In real world, an intensity dynamic range is 108:1, and the dynamic range that could be sensed by human eyes is quite large. The human eyes may perceive the starlight having the intensity of 10−3 cd/m2, and may also sense the sunlight having the intensity of 105 cd/m2. The human eyes may perceive every detail in the real scenes under quite bright and dark situations. It may be known that through the automatic adjustment of the human eye pupils, the intensity dynamic range capable of being recognized by the human eyes is approximately 108:1. Even in the same scene, the human eyes may recognize the intensity dynamic range of 105:1 without any adjustment.
The intensity dynamic range of recent image capturing devices (such as digital image capturing devices and video cameras) is relatively low, so the relevant details cannot be seen in the especially bright scenes, and the relevant dark details cannot be captured in the especially dark parts. Therefore, the capturing device cannot match with the demand of the practical scene and the intensity dynamic range capable of being sensed by the human eyes. In the surveillance application, such as the surveillance on the express way, capturing at night, and capturing for sunrise and sunset, it includes the high light source, such as front and back lights of vehicles, street lights, sun, and surfaces with the high reflectivity, directly facing the lens of the image capturing device and the details having the low intensity, such as shadows, sceneries behind the light source, and other regions cannot be irradiated by the lights. Usually in these occasions, we have to adjust the exposure time. However, for some cases, it is impossible to capture all the details in the high light region or that in the low light region of the high contrast scene through adjustment. Therefore, it is necessary to provide various methods for enhancing the capability of capturing the details and the corresponding intensity dynamic range of the image capturing device.
The images having the high dynamic range fused by recently popular methods in the academic community, usually has to calibrate a response curve of the image capturing device by an optimizing manner. A high dynamic range (HDR) image is calculated according to the response curve of the image capturing device and an image sequence captured under different exposure situations. Visually, HDR image ranged from 0 to 105 or even larger is calculated by the image sequence with pixel values of 0-255. To display such HDR image on a common display device, a tone mapping operator are performed on the obtained HDR image (the 0-105 HDR image is mapped to a common 0-255 image).
In the prior art, a technical solution uses an image capturing device to capture the image. The exposure time of each frame is set according to the pixel intensity distribution before capturing. The images having the long and the short exposure time are captured alternate. The pixels between each frame and the neighboring frames are determined at the stage of initializing frame. The intensity range of the corresponding frame is calculated by a relevant pixel selected from the corresponding pixel. The intensity range of the frame is concerted into a common 8-bit image by the tone mapping.
After analyzing the prior art, the inventor finds some problems as follows.
In the prior art, it is necessary to frequently calibrate the response curve of the image capturing device, as the response curve changes with some environmental characteristics, for example, the temperature. During the tone mapping stage, the mostly popular method with the best effect is based on a self-adaptive tone mapping algorithm. However, this process may irreversibly damage the good physical characteristics maintained by the two complicated calibrations and the heavy computation mentioned above, for example, the intensity value of the image is proportional to the scene intensity and irrelevant to the exposure time etc. More, this is of a high computational complexity. Thus, it is difficult to implement as one realtime system, and the fused image is not satisfactory.